Common alkaline batteries include a potassium hydroxide aqueous solution as an alkaline electrolyte and particulate zinc as a negative electrode active material. However, zinc is susceptible to corrosion in the alkaline electrolyte. Thus, self-discharge reaction, i.e., dissolution of zinc from the zinc particle surface, occurs. When zinc dissolves, zinc ions and electrons are produced. On the zinc particle surface, the produced electrons react with protons in the electrolyte, thereby producing hydrogen gas. The hydrogen gas inside the battery increases the inner pressure of the battery, which may result in leakage of the alkaline electrolyte during storage.
The most effective method for suppressing the self-discharge of zinc is to use mercury for amalgamating zinc. Mercury is capable of completely covering the zinc particle surface and has a high hydrogen overvoltage. However, from the environmental consideration, other alternatives are currently used. A common alternative is a method of alloying zinc with other metal to improve its corrosion resistance. Examples of other metal include aluminum, calcium, bismuth, and indium. Another method is to dissolve zinc oxide or zinc hydroxide in an alkaline electrolyte to suppress the corrosion of zinc.
However, simply improving the corrosion resistance of zinc is not sufficient for reducing the production of hydrogen gas inside the battery. Hence, alkaline dry batteries are provided with a space into which hydrogen gas escapes, in order to suppress an increase in battery inner pressure due to the production of hydrogen gas.
To suppress the production of hydrogen gas inside the alkaline battery, Japanese Laid-Open Patent Publication No. Hei 7-6759 proposes coating the surface of a negative electrode current collector with a metal having a high hydrogen overvoltage. This publication also proposes adding an inhibitor to an electrolyte or a negative electrode active material. Japanese Laid-Open Patent Publication No. Hei 10-334904 and Japanese Laid-Open Patent Publication No. Hei 10-334906 propose using a zinc alloy containing bismuth, tin and the like and adding an inorganic inhibitor such as an indium oxide to the powder of the zinc alloy.
In the case of silver oxide batteries, a silver oxide, which is a positive electrode active material, catalyzes the oxidation of the hydrogen gas produced in the negative electrode to water, and this catalytic action slows the speed at which the hydrogen gas accumulates inside the battery. It is therefore possible to slow the increase of the battery inner pressure and prevent liquid leakage.
When the surface of a current collector is coated with a metal having a high hydrogen overvoltage, as proposed by JP No. Hei 7-6759, the production of hydrogen gas from the current collector surface is suppressed, but the production of hydrogen gas from the zinc alloy particles, which have a larger surface area than the current collector, cannot be suppressed. Hence, the production of hydrogen gas is not sufficiently suppressed.
Also, when an inorganic compound is added to an electrolyte or a negative electrode active material as an inhibitor, as proposed by JP No. Hei 7-6759, the so-called displacement plating reaction occurs on the zinc alloy particle surface. That is, the dissolution of zinc and the deposition of metal derived from the inorganic compound onto the particle surface occur simultaneously. At this time, on the zinc alloy particle surface, electrons are transferred to the protons in the electrolyte, so that the reaction of hydrogen gas production occurs competitively. As a result, the amount of hydrogen gas produced during an early stage of storage increases.
Further, when bismuth, tin and the like are contained in a zinc alloy and an inorganic inhibitor is added to the powder of the zinc alloy, as proposed by JP No. Hei 10-334904 and JP No. Hei 10-334906, the production of hydrogen gas is not sufficiently suppressed either.
Furthermore, due to the formation of a local battery between the negative electrode active material and the current collector, the dissolution reaction of zinc and the deposition reaction of metal on the negative electrode active material surface are promoted. Hence, the contact resistance between the negative electrode active material and the current collector increases and the internal resistance of the battery during storage significantly increases, thereby resulting in degradation of heavy load discharge performance, particularly pulse discharge characteristics.
When an organic compound is added as the inhibitor, the organic inhibitor covers the zinc alloy particle surface, thereby reducing the active site where hydrogen gas is produced and suppressing the production of hydrogen gas during an early stage of storage. The organic inhibitor covering the particle surface, however, interferes with the zinc reaction and therefore impairs the discharge characteristics. Further, the zinc alloy covered with the organic inhibitor is easily pulverized due to discharge, and if the organic inhibitor separates from the zinc alloy surface, it is unlikely to adsorb to the particle surface again. As a result, the amount of hydrogen gas produced from the particle surface increases as discharge proceeds.
As described above, when the displacement plating reaction occurs on the surface of the negative electrode active material or current collector, the reaction of hydrogen gas production occurs simultaneously, so that the amount of hydrogen gas produced during an early stage of storage increases. It is therefore desired to suppress the competitively-occurring hydrogen gas production reaction. Also, in order to suppress degradation of heavy load discharge characteristics, it is necessary to suppress an increase in the contact resistance between the negative electrode active material and the current collector.
Among alkaline batteries, button air batteries particularly have a difficulty in providing them with a space into which hydrogen gas escapes. If a space is provided inside the battery, the amount of negative electrode active material filled therein needs to be reduced, so that a sufficient discharge capacity cannot be obtained. As in silver oxide batteries, a button air battery may include a catalyst for oxidizing hydrogen gas to water in its positive electrode (air electrode), but the use of such a catalyst in the positive electrode results in a decrease in the positive electrode's ability to reduce oxygen and an increase in production costs.
In view of the above, the present invention intends to suppress the production of hydrogen gas inside a battery and to provide an alkaline battery that has excellent storage characteristics both when not used and when partially discharged.